The first question I had was whether either of the sizes had any objective benefits. A quick Google search for the string
A4 vs letter revealed a few answers. The most relevant answer I found is on the following link:
The width to length ratio of the A4 allows it to be folded into half without altering the ratio. Apparently this is quite important. Yet the US (and a few other countries) are hanging on to the letter size.
In my personal opinion the letter size is more elegant as compared to the A4 size. After having used the A4 size for most of my life, the change to the letter size makes it seem to me to be easy to hold and a bit more easier on the eyes. The A4 is tad bit too long. That's a subjective judgement. I am looking for the objective benefits, if there are any, for using the letter size (apart from the legacy benefits of course).